Speedo calibration

I fitted some new glow dials in my V6 and I don't think they are measuring the speed properly. I seem to alsways be doing 40mph on my speedo when I keep up with the car in front on a 30mph road. I went through one of those digital signs near my house to test it out and when my speedo was reading 40 it said I was doing 32mph. any idea how to re-calibrate it?

nope its definatly not sticking as the person who owned the car previously had put in dials from like a fiesta or something and they were all bent anyways so I took them out there is only the thickness of the glow dials. its is reading above what it should be. not sticking anywhere.